A switching power source, which is used in portable small-size electronic apparatuses and which switches a dc obtained by rectifying and smoothing a commercial ac or a dc from a battery by using a high frequency of, for example, approximately several hundreds kHz so as to convert it to a desired voltage efficiently by using a small-size transformer, has been widely used.
As a typical construction of such a switching power source, a switching power source of a pulse-width modulation (PWM) system has been widely used, in which the secondary-side output voltage is detected by a voltage detection circuit, and a control circuit controls the switching pulse width of the main switching element in accordance with the results of detection so as to obtain a desired secondary-side output voltage.
Moreover, as another typical construction of the switching power source, a switching power source of a ringing choke converter (RCC) system has been widely used, in which excited energy, which has been accumulated in a transformer during the on-period of the main switching element, is outputted to a secondary-side circuit during the off-period, and upon completion of the output, a ringing pulse generated in the control coil of the transformer is fed back to the control terminal of the main switching element through a dc-cut capacitor so that the main switching element is again activated.
In the above-mentioned switching power source of the RCC system, as the load becomes higher, the above-mentioned off-period and on-period are automatically lengthened, that is, the switching frequency is reduced, so that the secondary-side output voltage is maintained at a predetermined constant voltage; therefore, a complex control circuit, such as required for the switching power source of the PWM system, is not necessary, and a power-source circuit for generating a voltage forming the basis for the pulse width also is not necessary, both contributing to achieve a low-cost power source.
FIG. 27 shows an electric circuit diagram of a typical prior-art switching power source 1 of the RCC system. A dc, obtained by rectifying a commercial ac by a main power-source circuit not shown, is inputted between input terminals p1 and p2. This dc is smoothed by smoothing capacitor c11, and a main power-source voltage is outputted between a main power-source line 2 on the high-level side and a main power-source line 3 on the low-level side from smoothing capacitor c11.
A series circuit consisting of the primary coil nil of a transformer n and a main switching element q is connected between the above-mentioned power-source lines 2 and 3. The above-mentioned main switching element q is realized by, for example, a bipolar transistor and a field-effect transistor, and FIG. 27 shows a case in which a field-effect transistor is used. A starter circuit 4, which consists of voltage-dividing resistors r3 and r5, is connected between the main power-source lines 2 and 3.
Upon application of power, that is, when a power-source voltage is applied between input terminals p1 and p2, the output voltage of smoothing capacitor c11, that is, the main power-source voltage, increases, and when the voltage-divided value due to voltage-dividing resistors r3 and r5 becomes not less than the threshold voltage of the main switching element q, for example, not less than 3 V, the main switching element q is turned off; thus, a voltage in the upward direction in FIG. 27 is applied to the primary coil n11 so that excited energy is accumulated therein. When the main switching element q is turned off in a manner as described later, a voltage in the upward direction is induced in the secondary coil n21 by the above-mentioned excited energy. Moreover, vibration, generated by leakage inductance between the primary coil n11 and the other coils n21 and n12 at the time of turning off, is absorbed and eliminated by a snubber circuit 5 that consists of a series circuit of resistor r11 and capacitor c12 and that is parallel-connected between the drain and source of the main switching element q.
The dc, induced in the above-mentioned secondary coil n21, is given to smoothing capacitor c13 through diode d12, and after having been smoothed by smoothing capacitor c13 , it is outputted to a load circuit, not shown, from output terminals p3 and p4 through the output power-source lines 6 and 7. A voltage detection circuit 8 is interpolated between the above-mentioned output power-source lines 6 and 7. The voltage detection circuit 8 is constituted by voltage-dividing resistors, photo-coupler pc1, etc., and light-emitting diode d13 of the photo-coupler pc1 is driven so as to light up with a luminance corresponding to the output voltage, and the value of the output voltage is fed back to the primary side.
Upon turning the main switching element q on, a voltage is induced in the control coil n12 in the same upward direction as that in the primary coil n11, and its induced current is given to the gate of the main switching element q through capacitor cl for cutting dc and bias resistor r2; thus, the gate potential of the main switching element q is further raised so that the main switching element q is maintained in the ON state.
Moreover, the current induced in the control coil n12 upon turning the main switching element q on is given to one of the terminals of capacitor c14 from capacitor c1 and bias resistor r2 through photo-transistor tr11 of the above-mentioned photo-coupler pc1. The other terminal of capacitor c14 is connected to the aforementioned main power-source line 3 in the low level; therefore, the higher the secondary-side output voltage becomes, the greater the charging current, thereby allowing the terminal voltage of capacitor c14 to increase rapidly. The charging voltage of capacitor c14 is supplied to the base of control transistor tr12 that is interpolated between the gate and source of the main switching element q, and when the output voltage goes beyond the threshold voltage of control transistor tr12, for example, not less than 0.6 V, control transistor tr12 is allowed to conduct, making the gate voltage of the main switching element q drop abruptly, with the result that the main switching element q is off-driven.
Therefore, the higher the secondary-side output voltage becomes, that is, the lighter the load, the quicker the charging voltage of capacitor c14 increases, with the result that the main switching element q is off-driven more quickly. Moreover, the current induced in the control coil n12 is supplied to capacitor c14 through resistor r12. The series circuit of these resistor r12 and capacitor c14 is connected in parallel with control coin n12 so as to form an overcurrent protection circuit. With this overcurrent protection circuit, even if the output voltage of smoothing capacitor c13 on the secondary side is low due to shortcircuiting between output terminals p3 and p4, etc., the on-time of the main switching element q is limited to a predetermined period, thereby making it possible to protect the main switching element q.
Here, supposing that the numbers of coil of the control coil n12 and the secondary coil n21 are represented by the same numbers as the reference numerals and the output voltage on the secondary side is vo, the voltage (n12/ n21) vo is induced in the control coil n12 in the downward direction of FIG. 27 upon turning the main switching element q off; thus, since the induced current is allowed to flow resistor r12, the charge of capacitor c14 is drawn, and a resetting operation for the next on-operation of the main switching element q is carried out.
When, after turning the main switching element q off, the excited energy, accumulated in the primary main coil n11, has been outputted to the secondary side, ringing occurs between a parasitic capacity c15 mainly possessed by the control coil n12 and the control coil n12, the electrostatic energy, accumulated in the parasitic capacity c15 with the voltage (n12/ n21) vo, is discharged, this is converted to excited energy of the control coil n12 after a 1/4 period of vibration, and then an electro motive voltage with the voltage (n12/ n21) vo in the upward direction is generated in the control coil n12 so as to again charge the parasitic capacity c15. The electro motive voltage, which is a ringing pulse, is set to be not less than the threshold voltage Vth of the main switching element q; thus, the main switching element q is again turned on by the electro motive voltage. In this manner, the main switching element q is continuously on/off-driven based upon the switching frequency corresponding to the load automatically, thereby making it possible to provide a desired secondary-side output voltage.
In the switching power source, most of losses are caused by power required to draw electric charge accumulated in the parasitic capacity between the drain and source of the main switching element and core losses in the transformer, and these losses generally increase as the switching frequency becomes higher. Therefore, as described above, in the switching power source 1, since the switching frequency becomes higher as the load becomes lighter, the ratio of losses with respect to converted power increases as the load becomes lighter, resulting in a problem of reduction in power conversion efficiency.
As other conventional techniques for solving the above-mentioned problem, for example, Japanese Laid-Open Patent Publication No. 47023/1997 (Tokukaihei 9-47023) and Japanese Examined Utility Model Publication No. 3039391 are listed. The conventional technique described in Japanese Laid-Open Patent Publication No. 47023/1997 has a construction in which: another control transistor is installed in parallel with the control transistor for turning off the main switching element; in the case of light load, an induced voltage, generated in the control coil upon turning off the main switching element, is instantaneously taken in a capacitor through the transistor that is turned off in response to the main switching element; and the other control transistor is turned on by the capacitor so that the off state of the main switching element is maintained so as to reduce the switching frequency.
Therefore, a complex construction is required for reducing power consumption, resulting in high costs as well as failing to utilize advantages of the RCC system; consequently, the charging process for capacitors becomes dependent on the storage time of the transistors, resulting in great dispersions between the devices and difficulties in designing.
Moreover, the conventional technique described in Japanese Examined Utility Model Publication No. 3039391 has an arrangement in which a delay capacitor for rounding a ringing pulse is interpolated in parallel with the control transistor at the time of light load.
Therefore, as described on column 0025, lines 7-8 in the above-mentioned official gazette, the switching cycle is extended only for the period in which ringing is occurring, with the result that it is not possible to greatly reduce the switching frequency at the time of light load, as compared with the switching frequency at the time of heavy load.
Moreover, even if the switching frequency at the time of light load is reduced to a great degree as compared with the switching frequency at the time of heavy load (at the time of normal operation) by using the conventional technique described in the above-mentioned Laid-Open Patent Publication No. 47023/1997 (Tokukaihei 9-47023), etc., since the rating of each constituent part of the switching power source has been selected based upon high-load operation, the following disadvantage might be raised.
FIG. 28 is a block diagram which schematically shows a conventional switching power source 1a which is disclosed in the above-mentioned Laid-Open Patent Publication No. 47023/1997 (Tokukaihei 9-47023) and which can reduce the switching frequency at the time of light load to a great degree as compared with the switching frequency at the time of heavy load. Here, since the above-mentioned disadvantage is raised regardless of constructions that reduce the switching frequency, FIG. 28 typically shows a control circuit 9 as a construction which reduces the switching frequency under conditions, such as a voltage reduction and light load. The control circuit 9 is connected between the main power-source lines 2 and 3 through starting resistor r3.
Moreover, the switching power source 1a is provided with the following construction so as to reduce the switching frequency in a stand-by state, that is, at the time of light load, of an apparatus in which the switching power source 1a is installed. A control signal is given to control terminal p5 from the above-mentioned apparatus side, a series circuit consisting of light-emitting diode d14 of photo-coupler pc2 and resistor r13 is connected between the above-mentioned control terminal p5 and an output power-source line 7 on the low-level side. Therefore, when the control signal goes high at the time of heavy load, that is, in a non-stand-by state, light-emitting diode d14 lights on, thereby informing the primary side of the heavy load state.
On the primary side, photo-transistor tr13 of the aforementioned photo-coupler pc2 is installed in the control circuit 9, and at the time of the heavy load, photo-transistor tr13 is turned on so that the oscillation frequency restricting operation of the control circuit 9 is suspended, the ringing pulse is supplied to the main switching element q, with the result that the aforementioned normal RCC operation is carried out. In contrast, at the time of the light load, the control signal to control terminal p5 goes low, light-emitting diode d14 goes out, and control transistor tr12 is turned on; thus, an oscillation frequency restriction operation is carried out, control transistor tr12 is kept on so that the ringing pulse is bypassed, and after a lapse of a predetermined time, control transistor tr12 is turned off so that the main switching element q is turned on by a divided voltage value inside the control circuit 9 derived from the aforementioned starting resistor r3.
In this manner, the oscillation frequency at the time of light load is reduced, and power consumption, required for drawing an electric charge that has accumulated in a parasitic capacity between the drain and source of the main switching element q, and core losses in the transformer n can be reduced; thus, it is possible to improve the efficiency of power conversion.
Additionally, in the construction of Japanese Laid-Open Patent Application No. 47023/1997 (Tokukaihei 9-47023), the portion of the above-mentioned resistor r12 is constituted by a series circuit of a resistor and a Zener diode, and a resistor that is placed in parallel with the series circuit; thus, the higher the main power-source voltage, that is, the output voltage of smoothing capacitor c11, the greater the current flowing into the Zener diode, thereby compensating for changes in the output voltage. Therefore, in the present specification, for simplifying explanation, the main power-source voltage is made constant, and such a construction is replaced by resistor r12.
In the switching power source 1a having the above-mentioned construction, the on-period of the main switching element q is determined by the time during which the accumulated charge having a polarity reversed to that of FIG. 28 has been discharged and is again charged to 0.6 V having the polarity shown in FIG. 28.
However, at the time of heavy load, the above-mentioned charging time is a relatively long period of time during which, after the main switching element has been turned on, the accumulated charge having the reversed polarity is drawn and charging is made to provide the positive polarity.
In contrast, at the time of light load, the time during which, after control transistor tr12, which has bypassed the ringing pulse, has been turned off, the divided voltage value inside the control circuit 9 derived from starting resistor r3 increases so that the main switching element q is again on-driven is very long, that is, an operation suspension time for decreasing the oscillation frequency is provided; therefore, the accumulated charge having the reversed polarity inside the above-mentioned capacitor c14 is consumed by resistor r12 and a control coil n12 during the operation suspension time so that the above-mentioned charging time becomes shorter as compared with that at the time of heavy load.
Therefore, at the time of light load, the current limitation value of the overcurrent protection circuit becomes smaller as compared with that at the time of heavy load, with the result that energy to be accumulated inside the transformer n becomes smaller; this fails to lower the switching frequency sufficiently so as to supply an amount of power required to the secondary side.
In this respect, it is possible to reduce the switching frequency as has been initially aimed by using parts having high current ratings as the respective constituent parts so as to raise the current limitation values in the two light-load and high-load operation modes as a whole; however, this is not preferable from the view point of costs.
Additionally, in FIG. 28, the explanation has been given by exemplifying a case in which, with respect to the load, the load of the switching power source 1a itself gives a control signal for indicating whether or not it is in a light load state to the switching power source 1a; however, the switching power source may be designed to judge whether the load is light or heavy.
FIG. 29 is an electric circuit diagram showing a switching power source 1b of this case. In this switching power source 1b, detection resistor rs, diode d21, capacitor c21, comparator a21 and reference voltage source e21 are installed on the secondary side, and the output of comparator a21 is given to a control circuit 9b from light-emitting diode d14 of photo-coupler pc2 through photo-transistor tr13. This detection resistor rs carries out a current-voltage conversion on a load current flowing through the output power-source line 7, and gives its terminal voltage to comparator a21 through diode d21 and capacitor c21. Here, comparator a21 monitors the load current by comparing the terminal voltage with the reference voltage vref derived from reference voltage source e21.
When the load current becomes greater, the terminal voltage of detection resistor rs becomes higher than the reference voltage vref so that comparator a21 outputs a high-load signal to the control circuit 9b through photo-coupler pc2, thereby allowing the control circuit 9b to carry out the normal RCC operation. In contrast, when the load current becomes smaller, the terminal voltage of detection resistor rs becomes lower than the reference voltage vref so that comparator a21 outputs a light-load signal to the control circuit 9b, thereby allowing the control circuit 9b to reduce the switching frequency. In this manner, even at the time of an excessive light load beyond the load variations in the normal operation, such as a stand-by state, the switching frequency is reduced so that it is possible to improve the power conversion efficiency.
In another conventional switching power source 1c, a circuit used for detecting the load condition is installed on the primary side as illustrated in FIG. 30. Specifically, the detection-use source current of the main switching element q is current-voltage converted by detection resistor rs that is series connected with the main switching element q, and the terminal voltage is given to comparator a21 through diode d21 and capacitor c21, and compared with the reference voltage vref from reference voltage source e21 so as to be monitored. Thus, when the source current of the main switching element becomes smaller due to a light-load state, comparator a21 is allowed to output a light-load signal to the control circuit 9c.
The switching power sources 1b and ic having the above-mentioned construction have a problem in which a comparatively large current flows through detection-use resistor rs with the result that there is a great power loss. Moreover, it is necessary to completely insulate the primary-side circuit and the secondary-side circuit in order to satisfy requirements on the safety standard; therefore, in the switching power source 1b, the construction for detecting the load condition, such as the aforementioned detection-use resistor rs, is installed on the secondary side, and photo-coupler pc2 is used to transmit the result of detection to the control circuit 4; this results in a problem of high costs.